U.S. Pat. No. 5,522,697 describes a known variable geometry turbine in which a turbine wheel is mounted to rotate about a pre-determined axis within a housing. An inlet passage to the turbine wheel is defined between a fixed wall of the housing and a sidewall which is displaceable relative to the fixed wall in order to control the width of an inlet passage. The sidewall is supported on rods extending parallel to the wheel rotation axis, and the rods are axially displaced relative to the housing so as to control the position adopted by the sidewall.
The rods are displaced by a pneumatic actuator mounted on the outside of the housing, the pneumatic actuator driving a piston. The actuator piston is coupled to a lever extending from a shaft pivotally supported by the housing such that displacement of the lever causes the shaft to turn. A yoke having two spaced apart arms is mounted on the shaft in a cavity defined within the housing. The end of each arm of the yoke is received in a slot in a respective sidewall support rod. Displacement of the actuator piston causes the arms to pivot and to drive the sidewall in the axial direction as a result of the interengagement between the arms and the sidewall support rods.
In a co-pending application with the same priority date as this application, a variable geometry turbine is described in which the external actuator mechanically coupled to the sidewall is replaced by a piston and cylinder arrangement within the housing. Problems have been experienced in controlling the axial position of the sidewall with both the conventional external actuator arrangements and arrangements relying upon a piston and cylinder within the housing. In particular, the sidewall has proved difficult to control as it approaches a fully closed position, that is a position in which the width of the turbine inlet passage is a minimum.